The present invention relates to a communicating system, a communicating method, a base station, and a mobile station applicable for an inter-vehicle communicating system for transmitting for example multimedia data from the base station to a vehicle and vice versa.
In the inter-vehicle communicating system, there are needs for multimedia communication. In the multimedia communication, mobile stations send requests for information to the base station so that it downloads multimedia data such as video data and music data to the mobile stations. The multimedia communication has a feature of which the amount of data that are transmitted (uplinked) from the mobile station to the base station are smaller than the amount of data that are transmitted (downlinked) from the base station to the mobile station. Thus, to accomplish the multimedia communication with the inter-vehicle communicating system, a radio communicating system that can effectively transmit a large amount of data that are generated as burst data to mobile stations is required. In addition, when photographed video data are transmitted from a mobile station to the base station, it cannot be always said that the amount of photographed video data is small. Thus, it is desired to effectively transmit video data.
To suitably perform inter-vehicle multimedia radio communication, a communicating method for forming both a frame of the uplink channel and a frame of the downlink channel with TDMA (Time Division Multiple Access) frames has been proposed (Patent Document 1).
[Patent Document 1] Japanese Patent Laid-Open Publication No. 2003-32171
Next, the communicating method described in the Patent Document 1 will be described with reference to FIGS. 31A and 31B. In FIGS. 31A and 31B, BS represents a base station (or an access point (AP)). T1 to Tn represent n mobile stations (MS). As shown in FIG. 31A, one frame of downlink channel transmitted through the downlink channel is composed of one notification slot (shaded slot) and one or a plurality of (for example, seven) data slots. A process for dividing a successive period at constant intervals is referred to as slot segmentation. Each portion of which a successive period is divided is referred to as a slot. In the notification slot, a notification packet is allocated. In the data slots, data packets are allocated. A notification packet and data packets transmitted from the base station are received by the mobile stations T1 to Tn.
A notification packet contains information of statuses of the data slots of the current frame of the downlink channel, information of statuses of the data slots of the subsequent frame of the downlink channel, information of statuses of the data slots of the current frame of the uplink channel, information of statuses of the data slots of the subsequent frame of the uplink channel, and information of the reception statuses of the base station against uploaded packets that the mobile stations have transmitted in the immediately preceding frame of the uplink channel.
FIG. 31B shows an example of the structure of a frame of the uplink channel. A frame of the uplink channel is composed of one or a plurality of (for example, five) data slots, at least one download request slot for a download request packet, at least one upload request slot for an upload request packet, and a plurality of acknowledgement (ACK) slots. ACK slots corresponds to seven data slots that are downloaded from the base station BS.
Although at least one download request slot and at least one upload request slot are needed, if the number of request slots is small, when each mobile station transmits a request packet to the base station BS at a time, the possibility of which the request packets transmitted from mobile stations collide becomes high. However, when the number of request slots is increased, the number of data slots is decreased. Thus, in the example shown in FIG. 31B, the number of download request slots and the number of upload request slots are set to 7 and 5, respectively.
Next, the relation between timing of a frame of the downlink channel and timing of a frame of the uplink channel will be described. When a download request packet and an upload request packet are transmitted, the base station needs to be able to generate notification packets of the subsequent frame against the request packets. In addition, ACK packets that represent information about the reception statuses of data packets of the frame of the downlink channel should be transmitted before the subsequent frame is uploaded so as to prevent the ACK packets from colliding with the subsequent frame. In addition, after the notification packet is received from the base station, the upload data packets and the request packets should be transmitted.
When a mobile station needs to download or upload, the mobile station transmits a request packet with any download request slot or any upload request slot of the subsequent frame to the base station BS through the uplink channel in accordance with the contents of which the base station has noticed.
When the base station receives a download request packet from a mobile station, the base station performs a data slot allocating process for a frame of the downlink channel. When the base station receives an upload request packet from a mobile station, the base station performs a data slot allocating process for a frame of the uplink channel. The base station generates a notification packet in accordance with the statues of the data slots of the current frame and the subsequent frame and the reception statuses of upload packets received in the immediately preceding frame of the uplink channel.
The base station transmits data packets with data slots of a frame of the downlink channel to a mobile station that has transmitted a download request packet to the base station. The mobile station receives a notification packet and data packets from the base station and transmits the reception statuses thereof with an ACK slot to the base station.
A mobile station that has transmitted an upload request packet receives a notification packet from the base station. The mobile station transmits data packets to the base station with an allocated data slot. When the base station has completely transmitted data packets to a mobile station and has completely received data packets from the mobile station, the base station unallocates the data slot assigned to the mobile station.
While the base station is transmitting data packets to a mobile station, if the base station receives a request packet from another mobile station, the base station allocates one data slot to each mobile station and the other blank data slots to each mobile station in accordance with a predetermined rule for the subsequent frames. An example of the predetermined rule is as follows.
When the base station receives download requests from mobile stations, the base station allocates all blank data slots to a mobile station whose data packets to be transmitted are the smallest in these mobile stations. When the base station receives upload requests from mobile stations, the base station allocates all blank data slots to a mobile station whose data packets to be received are the smallest in these mobile stations. When there are still blank data slots, the base station allocates these data slots to a mobile station whose data packets to be received are the next smallest in these mobile stations. The base station performs the allocating process until there is no blank data slot or there is no mobile station to assign a data slot.
Next, the allocating rule for data slots that are downloaded will be described in detail. Likewise, this rule can be applied for data slots that are uploaded. The rule consists of a first rule and a second rule. The first rule describes that while the base station is transmitting data packets to a particular mobile station, when the base station does not receive a download request packet from another mobile station (at least one mobile station), the base station allocates all blank data slots to the particular mobile station. The second rule describes that while the base station is transmitting data packets to a particular mobile station, when the base station receives a download request packet from another mobile station (at least one mobile station), the base station allocates one data slot of each of the subsequent frames to each mobile station that has transmitted a request packet to the base station, allocates blank data slots of each of the subsequent frames to a mobile station whose data packets to be transmitted are the smallest in these mobile stations, and when each frame has blank data slots, allocates the blank data slots to mobile stations whose data packets to be transmitted are smaller than these mobile stations. The base station performs the process in accordance with the second rule until there is no blank data slot or there is no mobile station to allocate a data slot. The second rule can have several modifications. The data slot allocating process for frame of the uplink channels is the same as the data slot allocating process for frames of the downlink channel.
When the base station notifies a mobile station that the subsequent frame has a blank slot, the mobile station transmits a download request packet or an upload request packet to the base station with any download slot or any upload slot.
When a mobile station receives a notification packet from the base station, if the notification packet is a download request packet, the mobile station determines whether or not the subsequent frame of the downlink channel has a blank data slot in accordance with the notification packet. When a mobile station receives notification packet that is a upload request, the mobile station determines whether or not the subsequent frame of the uplink channel has a blank data slot in accordance with the notification packet. At this point, if the mobile station has determined that the subsequent frame of the uplink channel does not have a blank data slot, the mobile station does not transmit the request packet, but just stands by.
When the determined result represents that the subsequent frame of the uplink channel has a blank data slot, the mobile station allocates any request slot for a request packet and transmits the request packet with the allocated request slot. The mobile station transmits a download request packet to the base station with a download request slot. Likewise, the mobile station transmits an upload request packet to the base station with an upload request slot.
The mobile station receives the subsequent notification packet. The mobile station determines whether or not the base station has received the request packet in accordance with the information of the received notification packet. When the determined result represents that the base station has received the request packet that is a download request packet, the mobile station receives a data packet from the base station. When the mobile station has completely received the data packet of the frame, the mobile station transmits an ACK packet that represents information of the reception status with an ACK slot. After the mobile station has completely received all data packets, the mobile station stands by.
When the request packet is an upload request packet, the mobile station transmits data packets to the mobile station with a data slot of a frame of the uplink channel. When the mobile station has completely transmitted all data packets to the base station, the mobile station stands by. When the base station has completely received all data packets from the mobile station, the base station disallocates the data slot allocated to the mobile station.
According to this communicating method, when the base station downloads data packets to a mobile station, the base station can allocate at least one data slot of one frame to the mobile station. Thus, this method has a higher throughput than the method for allocating one data slot of one frame to one mobile station. Likewise, when a mobile station uploads data packets to the base station, it can allocate at least one data slot to the mobile station. As a result, a mobile station can upload a large amount of data to the base station in a short time.
However, according to the communicating method of the Patent Document 1, ACK slots are allocated in each frame of the uplink channel. When a mobile station has completely received data packets of a frame of the downlink channel, the mobile station transmits an ACK packet to the base station. In this system, although data communication can be safely performed, since the mobile station side needs to have a buffer having a large capacity for storing data packets of the frame and needs to perform a complicated process for transmitting an ACK packet.
To solve such a problem, another communicating method has been proposed. In this method, an ACK slot is not allocated in each frame. When data such as an EOD (End Of Data) of a sequence of data of downlink channel are received, an ACK (with a data slot) is sent back to the base station. This communicating method has been proposed in Patent Document 2
[Patent Document 2] Japanese Patent Laid-Open Publication No. 2003-234688
According to the communicating method of the Patent Document 2, with the benefit of the Patent Document 1, a large amount of data can be communicated in a relatively short time, the mobile station side can simplify the communicating process and decrease the circuit scale. In this method, a response signal is sent back as a block of a predetermined amount of data. Thus, the normal transmitting process becomes the same as the re-transmitting process. As a result, the storage capacity of the buffer on the mobile station side can be decreased. In addition, the process can be simplified. In this example, the block of data represents for example an IP packet transmitted through the Internet or the like. Data are basically communicated in accordance with the TCP/IP.
Because the so-called MAC layer protocol has been improved, multimedia communication between vehicles has become more effective than before. However, it cannot be said that such communication is sufficient from a point of view of transmission and reception of multimedia data. It is essential to further improve the transmission rates and transmission quality without need to enlarge frequency bands.
Since transmission rates of inter-vehicle multimedia radio communication are 100 Mbps or higher, a frequency band of around 100 MHz should be secured. To have as a large cell radius as possible, a frequency band ranging from 3 GHz to 10 GHz microwave band will be mainly used. However, so far, the frequency band has become tight. To enlarge the communication area, it is necessary to take effective countermeasures against interference and suppress the frequency band as much as possible.
In addition, the mobile communicating system needs to provide effective countermeasures against multi-path interference. This leads to the improvement of transmission rates and transmission quality. In addition, considering connectivity with the IP network, as one possible solution, a communicating system that accomplishes the MAC layer protocol described in the Patent Document 2 could be accomplished on the basis of the Orthogonal Frequency Division Multiplexing (OFDM).